If a user has capital letters in his/her email address, then s/he cannot log it. Example: If I use Shawn.Kwang@ligo.org as my email address, then I cannot login with either email 'Shawn.Kwang@ligo.org' or 'email@example.com'.
I feel this is relatively new, and may be a regression caused by a recent change. The issue may require further investigation.
FTR: I believe SMTP doesn't care about upper/lower case letters in an email address, but Drupal/BOINC and our database is case-sensitive.